<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<style>
    ul>li{
        float: left;
        margin-left: 20px
    }
    div{
        display: none;
    }
    ul>li:hover div{
        display: block;
    }
</style>
<body>
    <ul>
        <li>aaa
            <div class="aa">aaaaaaaaaaaaa</div>
        </li>
        <li>bbb
            <div class="aa">bbbbbbbbbbbb</div>
        </li>
        <li>ccc
            <div class="aa">cccccccccc</div>
        </li>
        <li>ddd
            <div class="aa">ddddddddddd</div>
        </li>
    </ul>
</body>
</html>